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Regex.Replace Method (String, String, Int32)

Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.

Namespace:  System.Text.RegularExpressions
Assembly:  System (in System.dll)

public string Replace(
	string input,
	string replacement,
	int count


Type: System.String
The string to search for a match.
Type: System.String
The replacement string.
Type: System.Int32
The maximum number of times the replacement can occur.

Return Value

Type: System.String
A new string that is identical to the input string, except that a replacement string takes the place of each matched string.


input or replacement is null.

The search for matches starts at the beginning of the input string. The regular expression is the pattern that is defined by the constructor for the current Regex object. If count is negative, replacements continue to the end of the string. If count exceeds the number of matches, all matches are replaced.

The replacement parameter specifies the string that is to replace the first count matches in input. replacement can consist of any combination of literal text and substitutions. For example, the replacement pattern a*${test}b inserts the string "a*" followed by the substring that is matched by the test capturing group, if any, followed by the string "b". The * character is not recognized as a metacharacter within a replacement pattern.


Substitutions are the only regular expression language elements that are recognized in a replacement pattern. All other regular expression language elements, including character escapes, are allowed in regular expression patterns only and are not recognized in replacement patterns.

The following example replaces the first five occurrences of duplicated characters with a single character. The regular expression pattern (\w)\1 matches consecutive occurrences of a single character and assigns the first occurrence to the first capturing group. The replacement pattern $1 replaces the entire match with the first captured group.

using System;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string str = "aabccdeefgghiijkklmm";
      string pattern = "(\\w)\\1"; 
      string replacement = "$1"; 
      Regex rgx = new Regex(pattern);

      string result = rgx.Replace(str, replacement, 5);
      Console.WriteLine("Original String:    '{0}'", str);
      Console.WriteLine("Replacement String: '{0}'", result); 
// The example displays the following output:
//       Original String:    'aabccdeefgghiijkklmm'
//       Replacement String: 'abcdefghijkklmm'

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.